The present invention relates to the use of a spectroscopic method (NIR (Near-infrared spectroscopy)) for monitoring an industrial process (production of resin bonded composites particularly formaldehyde resin bonded composites) from the analysis of raw materials and intermediate products (such as urea-formaldehyde concentrates) to the quality control of the final product.
The use of NIR spectroscopy to monitor various processes has been developed systematically during the last years and was assisted by the increase in speed and capacity of the computers available. This NIR technique has been applied to various industries such as the oil industry, pharmaceuticals, food industry, control of fermentation, and certain polymer manufacture. There has been the use of the system in on line and end point determinations and reaction coordination for homogeneous and heterogeneous reactions. The analysis has been carried out on liquid and vapor phase process streams. In line process monitoring on polymer systems by NIR spectroscopy is discussed by D. Fischer et al., Fresenius J. Anal. Chemistry, 359 (1997) page 74 and J. Dunkers et al, Fourier Transform-NIR monitoring of Reacting Resins Using an Evanescent Wave High Index Fiber optic Sensor, Applied Spectroscopy, 52 (4), 1998, page 552. In particular the use of the system has been described in particleboard (composite board) manufacturing for monitoring raw wood quality (B. Engstrom and Mona Hedqvist, Prediction of the Properties of board by using a spectroscopic method combined with multivariate calibration, U.S. Pat. No. 5,965,888).
The manufacturing of composite wood-based panels (particleboard, medium density fiberboard, etc.) originated from a market need to provide inexpensive wood product alternatives and relied originally on the use of urea-formaldehyde adhesives. At high formaldehyde to urea ratios (F/U≈1.5 molar), these water-based adhesives are easy to make and use, and give excellent bonding results to almost any kind of wood chip. Around 1978, environmental concerns for formaldehyde emission imposed lower F/U ratios (≈1) that brought up the need for a much more careful and systematic control of the adhesive production. Furthermore, it was proposed that urea-formaldehyde concentrate (UFC) be used instead of formalin as a raw material for the preparation of the adhesives, in order to reduce the costs and hazards of transportation and to avoid the application of vacuum for the distillation of the excess water at the end of resin production. Despite its merits, only very few manufacturers implement today the UFC approach, because of its chemical complexity and the lack of quick methods for its characterization. Formaldehyde-based adhesives are made reliably by relatively large companies that have developed semi empirical know-how and can afford occasional application of costly and time consuming off-line monitoring techniques (GPC, NMR, etc.).
UFC is an intermediate for the resin synthesis that is typically prepared, by a continuous process, in an absorption tower. During this process gaseous formaldehyde is absorbed by an aqueous solution of urea. Absorption involves both dissolution of formaldehyde as well as chemical reaction of formaldehyde with urea. The ratio of urea to formaldehyde and the total solids content, pH, and temperature vary along the absorption tower and are important for the quality of the final product and the safe continuous production. Irregularities in the process can result in insoluble precipitate formation along the length of the tower or even blocking of intermediate disks of the tower.
The final product is a complex mixture of at least fifteen different compounds. The precise determination of the urea and formaldehyde content in these compounds is essential for the subsequent formulation of the resin. Conventionally it is performed only off line by tedious methods. One such method involves the hydrolysis of the UFC to obtain formalin that is subsequently extracted and after numerous dilutions; its concentration is finally determined by titration. The concentration of urea is calculated independently by determining the total nitrogen concentration (Kjeldahl method). The overall determination of formaldehyde and urea content with these methods takes more than four hours. Faster chromatographic methods have been developed but they are less accurate and require precise sample weighting and specific equipment. Furthermore, they cannot be applied on line. The above methods can only determine the overall content of urea and formaldehyde and give no information on the existing chemical speciation.
The UFC produced is subsequently used for the resin production (conventionally performed in a batch process). (Alternatively, formalin, that does not contain any urea, can be used for the resin production). The process of resin production is influenced by the raw materials used and the conditions applied and particularly the pH and concentration of the various components at every particular time. Failing to terminate the reaction at the correct conversion level can result in crosslinking of the resin and formation of an insoluble network inside the reactor. Furthermore, variability of resin production can result in variations in resin""s performance that are decreasing the reliability to the customers.
An objective of the invention is to provide a methodology for the control of all the raw materials and intermediate products (methanol, formaldehyde, urea, urea solutions, UFC, melamine, etc.) involved in formaldehyde based resin synthesis.
Particularly for the case of UFC the objective of the present invention is to provide a methodology for the fast and reliable determination of its content in urea and formaldehyde. Furthermore this methodology will be adaptable to on-line monitoring of the UFC production process. Therefore the urea and formaldehyde content will be measured continuously and at various points along the absorption tower in order to ensure regular production or detect irregularities.
Another objective of the invention is to provide a methodology for monitoring the resin production and for ensuring the reproducibility of the final product.
It has surprisingly been found that NIR can be used for the determination of the overall content of urea and formaldehyde in UFC even though the latter is a complex mixture of more than 15 different compounds containing urea and formaldehyde.
It has also been surprisingly found that NIR can be used for the monitoring of reactions of urea and formaldehyde in the production of a UF resin. This enables the monitoring of the start of methylolation through to the ending of polymerization so as to an evaluation of the various stages of the production. Again it was surprising to find that NIR could be used for the monitoring of such a polymerization despite the complex mixture of different compounds formed during the polymerization.
According to a first aspect of the invention there is provided a method for controlling the production of formaldehyde resin compositions in which formaldehyde takes part in a reaction with one or a combination of co resin forming material (of the type phenol, urea, melamine) the method comprising monitoring at least one of the formation of reaction mixture and the course of the reaction by near-infrared (NIR) spectroscopy and adjusting the course of the reaction (when necessary) in accordance with the results of the spectroscopy to obtain optimum conditions for the reaction.
According to the second aspect of the invention there is provided a method for controlling the production of formaldehyde resin compositions in which formaldehyde takes part in a reaction with one or a combination of co resin forming material, the method comprising monitoring at least one of the formation of the reaction mixture and the course of the reaction by near-infrared (NIR) spectroscopy, adjusting the course of the reaction (when necessary) in accordance with the results of the spectroscopy to obtain optimum conditions for the reaction wherein said monitoring is effected by comparing near infra red spectra obtained periodically from the reaction mixture or data computed therefrom with measurements or data calculated previously during calibration of the system.
According to a third aspect of the invention there is provided a method for assessing the performance of formaldehyde-based resins, the method comprising subjecting the resin to near-infrared spectroscopy to determine the spectra of the resin and comparing the spectra so determined to reference spectra of resins of known performance.
The accuracy of the above determination is surprising in view of the fact that the region of the spectra used in the analysis contains combination bands that are influenced by the strong hydrogen bonding interactions known to be present.
It was also surprising that a batch polymerization process could be monitored from the beginning to the end with a single optical path, even though the cloudiness changes drastically during the process.
The methodology developed involves the use of a Fourier-Transform NIR (FT-NIR) spectrometer, in a fiber optic acquisition mode. A typical procedure involves the selection of the optical resolution and acquisition time in order to allow for the optimum accuracy and signal to noise ratio with the minimum acquisition time. For the acquisition of the spectra the probe has to be immersed in the sample that is measured. Customized software can allow for automated acquisition of spectrum, data treatment, and results analysis and display. When on-line measurements are performed spectra acquisition can be programmed to take place at specified intervals of time.
Thus one embodiment of the invention is a method in which the reaction is between formaldehyde and urea and the monitoring is carried out during the formation of resin by reaction of formaldehyde and urea with a view to optimizing the formation of the final resin.
A particular embodiment of the invention is a method in which the spectroscopy measure is applied to the preparation of urea formaldehyde concentrate (UFC) to provide a high quality intermediate for urea formaldehyde resin synthesis.
The control of the invention can be applied to various formaldehyde reactions of the resin forming components. Thus the reaction can be applied not merely to reactions of formaldehyde and urea but the reactions of formaldehyde and phenol or melamine. The nature of the secondary component would be immediately apparent to those skilled in the art since the manufacture of formaldehyde resins is well known and the nature of the other reacting component is also well known.
The particular embodiment involving application and preparation of UFC can be effected in an absorption tower wherein gaseous formaldehyde is absorbed by an aqueous solution of urea.
For example for the determination of the urea and formaldehyde content of UFC calibration is necessary. For this purpose, a chemometric algorithm can be used, based on the original spectra or on their derivatives. The method must be built on a database of a significant number of UFC samples for which formaldehyde and urea contents must be measured independently. More than one spectrum from each sample can be obtained at a specified temperature. The selection of the temperature depends on the specific application of the method that is developed. However, it was found that it is important to perform all measurements at the same temperature. Validation of the method involves removing each spectrum from the database and treating it as an unknown in order to quantify the accuracy of the prediction. The sample""s content in urea and formaldehyde can be determined using the remaining spectra. The resulting root mean square error of the estimation for the database has to be lower than 0.5 for both the urea concentration and the formaldehyde concentration. Typical application (see example 1) gives much better RMSEP (Rout Mean Square Error of Prediction) than this. Furthermore a conformity test will be provided in order to ensure that the UFC does not deviate from its statistical composition (not just the overall urea and formaldehyde content but also the specific ingredients). A similar method can be developed for on line control. In this case spectra acquisition can be performed automatically at specified intervals of time, for instance every 15 minutes or less.
As mentioned the process can also be used for monitoring resin synthesis for example reaction of formaldehyde and urea.
In another specified embodiment the near infrared spectroscopy monitoring is effected on a urea/formaldehyde reactor to define a pathway for formation of a urea/formaldehyde resin with adjustment of any deviations found by the spectroscopy when these are greater than the defined optimum pathway.
For further monitoring the process of resin synthesis, a probe should be installed in the reactor and acquisition of data should be performed at short time intervals (for instance every 2 minutes). Algorithms can be created for monitoring applications. One such algorithm, describes the system in a multidimensional vectorial space, assigns arbitrary values to at least two extreme situations encountered during the synthesis process in order to create a scale and interpolates any intermediate spectrum within these two or more standards. Other algorithms describe quantitatively band intensities or the position of frequency extremes specifically corresponding to reactants, intermediates, final products, or unwanted by-products. The time evolution of these scores allows the phenomenological monitoring of each reaction. Statistical evaluation of the time evolution of these scores for each type of resin synthesis allows for defining the xe2x80x9cpathwayxe2x80x9d that should be followed in a particular resin synthesis and of the maximum deviations from this pathway that still lead to the acceptable final product. Deviations larger than those defined as maximum are not acceptable and therefore result in a warning signal for the operator.
Thus the process of the invention can be used to monitor the production of starting materials such as urea formaldehyde concentrates for the use of raw materials, urea and formaldehyde for such UFC in the production of the final resin. Similarly the process is equally easily applicable to production of intermediates and final resin production for such resins as melamine formaldehyde and phenol formaldehyde resins. As stated earlier, it was surprising to find that FT-NIR spectroscopy could be used in such complex systems despite the use of FT-NIR spectroscopy for monitoring other types of reactions.
The course of the reaction can be adjusted as follows dependent on the prevailing conditions:
a) An undesirable fast initial rate of methylolation in the synthesis of UF can be observed with NIR and corrected by adjusting the pH;
b) An undesirable fast polymerisation can be observed by NIR and corrected either by increasing the pH or decreasing the temperature;
c) An undesirable slow polymerisation can be corrected by decreasing the pH or increasing the temperature; and
d) Incomplete methylolation can be detected by NIR and adjusted by prolonging the methylolation stage or increasing the temperature.
The above examples are given by way of example and are not nor are they intended to be exhaustive.
The adjustment process can be automated if required. Here the NIR readings are used to control various physical parameters such as acid and/or alkali additions to the resin constituents to control pH and burners or electrical heaters to control the temperature of the resin composition.